This invention relates generally to sensors for monitoring the dispensing of material onto a substrate, and more particularly concerns an infrared sensor for monitoring energy radiating from a material dispensed onto a substrate.
In applying coating materials such as paints and adhesives to substrates, it is often important to monitor the material to ensure that it has been suitably applied. Applied materials can be monitored visually, or through the use of various types of sensors.
In some applications sensors are employed which monitor energy radiant from the material applied to the substrate. For example, in the case of dispensing a heated adhesive bead onto a substrate, an infrared sensor can be employed to monitor the applied adhesive. Such an infrared sensor typically exhibits a resistance which is dependent upon the intensity of received infrared radiation that is within a certain energy band. The sensor is usually connected to circuitry, in which resistance changes, in the sensor, are converted to voltage variations, which are in turn amplified to provide an output signal indicative of the level of infrared radiation from the adhesive bead. This infrared radiation level is a composite indication of the size of the adhesive bead and its temperature.
In the case of a continuously applied bead of heated adhesive, the sensor can be positioned near the adhesive dispenser, slightly downstream therefrom, and the electrical output signal of the associated circuit monitored to assure that the infrared radiation from the adhesive bead falls within an acceptable range.
Typically, the dispenser and its associated hoses are heated in order to maintain the adhesive at its appropriate application temperature. For a packaging grade adhesive, the application temperature is typically between 250.degree. F. and about 400.degree. F. The elevated temperature of the dispensed adhesive and/or the body of the dispenser increases the surrounding ambient temperature. This in turn may raise the temperature of the sensor to a point where its monitoring characteristics are reduced or completely impeded so as to prevent accurate monitoring of the received infrared radiation as an adhesive bead passes the sensor.
Another difficulty that arises from the environment in which the sensor must operate results from adhesive coating the lens of the sensor. As the dispensing of the bead of adhesive is terminated, adhesive filaments may form and become airborne. These filaments may attach themselves to the lens of the sensor, thus obstructing its view. Also, during dispensing an irregularity in the nozzle, such as caused by a partial clog, may cause the bead of adhesive to deviate from its intended path and coat the sensor.
Still another difficulty in detecting dispensed beads of materials arises in assuring that the sensor is aimed at the intended bead of material to be sensed. This is important due to the limited field of view of the sensor and its sensitivity of the sensor. For example, if multiple beads of adhesive have been deposited on the substrate, it may be important to assure that the proper bead is being sensed.